Baked Plaice, Turbot or Brill with potatoes, fennel and herb butter

450g potatoes, peeled and thinly sliced

175g onions, thinly sliced

½ fennel bulb, very thinly sliced

extra virgin olive oil, for drizzling

1 x 1-1.75kg fresh plaice, turbot, brill or other flat fish on the bone

flaky sea salt

freshly ground black pepper

For the herb butter:

110g butter, softened

4 tsp finely chopped mixed herbs (flat-leaf parsley, chives, fennel and thyme leaves)